Image amplification for laser systems

ABSTRACT

An optical image amplifier comprising a copper bromide laser ( 10 ) to provide a source of optical radiation, an optical amplifier ( 18 ) for amplifying the optical radiation received from the laser ( 10 ), an optical image forming device ( 16 ) located intermediate the laser ( 10 ) and optical amplifier ( 18 ) for forming an image to spatially modulate the output of the laser ( 10 ), and optical focusing means ( 22 ) located at the output of the optical amplifier for translating the image or a portion thereof to a surface to be illuminated ( 30 ). The system increases the efficiency of irradiation of photo-sensitive films, used for example in the manufacture of integrated circuits and printed circuit boards using laser direct imaging.

This invention relates to the intensity amplification of images and inparticular the use of copper bromide lasers to do so. In one particularembodiment such an arrangement is useable for laser direct imaging.

BACKGROUND

The science of laser amplification is known and a multitude ofscientific uses exists for such devices and in the industrial/commercialarena (optical fibre laser transmission for communications etc).

Use of lasers and in particular metal vapour and copper bromide lasersis also known, their use being equally diverse in the scientificindustrial/commercial and medical fields.

Copper bromide lasers are additionally known for their very high opticalbrightness gain (approx 10,000).

One application that appears particularly suitable for the use of alaser occurs in the relatively new field of Laser Direct Imaging (LDI).

Forming an image on a photo sensitive film referred to generally asphoto-lithography. It is commonly employed to expose patterns via a maskon to devices such as integrated circuits, integrated circuit masks,flat panel displays and printed circuit boards as well as application inthe printing industry.

A conventional photolithographic process begins with the coating of awork piece with a layer of photo-resist. Selected regions of thephoto-resist are acted upon by light shone (typically Ultra Violet (UV))through the prepared film or mask inducing a chemical reaction in thephotoresist. Either the illuminated regions or the non-illuminatedregions (depending on the type of photo resist used) are then removed bya chemical process to leave a patterned layer covering the portion ofthe surface of the device that is to remain after further chemicalprocessing to remove the uncovered layer portion.

The device is then subjected to a chemical process, such as etching toremove the uncovered portions of the upper layer of the work piece.

The photo chemical resistive layer is then cleaned off the device andtracks, pads, via hole surrounds and the like remain on the surface ofthe device.

A laser scanner is another device that can be used to expose light on toa surface. A scanner can position with great precision one or morefocused and intensity modulated laser beams as a series of scan linesover an area of a work piece being patterned. That precision depends onthe sharpness of the focus of the laser beam, the accuracy of modulationof the laser beam, the precision with which the laser beam moves acrossthe layer being patterned and the synchronisation between the modulationand movement of the laser beam. The power of the laser will affect thephotoresist accordingly. Essentially prior arrangements are a pixel bypixel serial scanning arrangement.

FIG. 1 shows an example of a prior art arrangement of a Laser DirectImaging (LDI) system used for illuminating a printed circuit board. Inthis prior art example, an ultra-violet (UV) argon-ion laser system iscoupled into an Acousto-Optical Modulator (AOM) where the continuouswave laser beam is in effect switched on and off (time modulated) in amanner determined by rasterized data supplied from a computer. The nowmodulated laser light passes through various lenses and is directed viamirrors, to a scanning arrangement. In the prior art example, scanningmotion is realised through the use of a rotating “polygon”. The polygonis a multi-faced rotating mirror assembly and its speed of rotationtogether with the available laser power determines the energy availablefor photochemical changes to affect the photoresist coating on thesurface of the work piece. After each linear scan, the board moves tothe next row of a predetermined array of linear scans.

The faster the photo sensitive resist reacts to illumination, the fasterthe polygon can rotate and the shorter will be the time it takes toilluminate a line and thus a work piece. Thus the sensitivity ofreaction of the resists, the polygon and AOM speed and laser outputpower become major factors in determining the productivity of prior artlaser direct imaging systems using available photo sensitive resists.

It is possible to split the beam into multiple paths so as to expose theboard along multiple lines or locations to further increaseproductivity.

The various optical systems used to deflect and manipulate the laserbeam from its original output path through the system include reflectiveand refractive elements.

At present it seems that no significant improvement in laser powers atwave lengths below 532 nm can be seen in the near future thus mosteffort is being put into developing high sensitivity, dry UV resistshaving 10 mJ/cm² or greater sensitivity. However, even this figure isonly achieved in special formulations that are expensive, have shortshelf life and a low contrast ratio. Exposure times become critical butincreasing exposure periods will lead to lower productivity.

All this must occur with the highest possible work piece throughput andminimal material wastage and defects. Finding a suitable combination oflaser frequency, power, resists and working rate that matches all of theabove characteristics is the challenge at hand for LDI systemsdesigners.

As will therefore become apparent, the LDI area is ripe for LDIapparatus that can provide a large number (many thousands) of pixels ina parallel rather than a serial format. An amplification system asproposed herein will assist in this regard. However, it should be notedthat the LDI application described in detail herein is an example only,and other applications for image amplification are not excluded from thescope of the invention generally disclosed herein.

BRIEF DESCRIPTION OF THE INVENTION

An optical image amplifier consisting of:

-   -   an optical radiation source consisting of an operating copper        bromide laser tube having a laser output;    -   an optical amplifier spaced from said optical radiation source        orientated so as to receive an output from said optical        radiation source;    -   an active optical transmission or reflective image forming        device located intermediate said optical radiation source and        said optical amplifier by which an image can be formed to        spatially modulate the laser output; and    -   an optical focusing means located at the output of said optical        amplifier for translating said image or a portion thereof to a        surface to be illuminated.

In an aspect of the invention the object to be illuminated is a printedcircuit board blank covered with a photo resist sensitive to said laseroutput.

In another aspect of the invention said optical focussing means producesan array of pixels, at least one pixel wide, representing at least aportion of said image.

In a further aspect of this invention said optical focussing meansproduces an area of radiation representative of a portion of said image.

In another aspect of the invention said active optical image formingdevice is a computer controlled liquid crystal array.

In yet a further aspect of the invention green or yellow radiationoutput from the optical amplifier is frequency doubled to produce UVradiation.

The specific embodiments of the invention will now be described in somefurther detail with reference to and as illustrated in the accompanyingfigures. These embodiments are illustrative, and are not meant to berestrictive of the scope of the invention. Suggestions and descriptionsof other embodiments may be included within the scope of the inventionbut they may not be illustrated in the accompanying figures oralternatively features of the invention may be shown in the figures butnot described in the specification.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an illustration of a prior art arrangement of a laser directimaging system;

FIG. 2 is the schematic of one embodiment of a laser imaging systemincorporating one embodiment of the amplification system for increasingthe intensity of an image; and

FIG. 3 is a schematic block diagram of the control electronics for theembodiment disclosed in FIG. 2.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In an embodiment of the invention a copper bromide (CuBr) laser is usedas the oscillator 10, as is pictorially represented in FIG. 2. Suchlasers are known for their relatively high power and excellent beamquality, and very high optical gain.

However, in this embodiment of the invention a relatively low poweroutput is only needed to be generated at 12 and directed to an opticalsystem 14 that further adapts the laser output to illuminate the activearea of the image forming device. The components and their arrangementin this part of the apparatus are well known to those skilled in theart.

The wavelength of the copper bromide laser output is 511 nm and 578 nm(a green and yellow laser) and can be designed to give low power (<5.5Watts) which is ideal because it will not destroy or distort the imagemodulation device 16. The spatially modulated beam now in the form of apartial image of the printed circuit board feature information is nowcollimated by the optical system 20 and amplified by the second lasertube 18 to form a high power image containing many thousands of pixelsof information each with enough energy to “expose” the printed circuitboard and initiate the desired chemical reaction.

It is thought to be the dearly novel aspect of this invention that theimage formation medium is positioned at the output of a relatively lowpower laser source so as to spatially modulate its output. An opticalamplifier 18 is then used to increase the modulated image intensity. Thepath of the radiation after passing through the image formation mediumis imaged into the amplifier 20. The components and their arrangement inthis part of the apparatus are well known to those skilled in the art.Such an arrangement is typically reflective and not refractive aspictorially represented in FIG. 2.

It should be noted that the use of LDI as an application is merelyillustrative of the useful combination consisting of a low power laser,dynamic image modulation device and optical amplifier that representsthe invention. The advantage such a combination provides in an LDIenvironment is a good illustration but other or similar advantages indifferent environments and other applications will be readily apparent.

The dynamic image device is preferably at this time, a Liquid CrystalDisplay (LCD) that is arranged to display a pattern which modulates thepassing laser light, by allowing or not allowing, the transmission ofthe low power laser light through it. The resolution of the image isdetermined not only by the laser divergence but is also determined bythe element size of the LCD display.

An LCD is but one example of a suitable dynamic display device and sincethey can be relatively slow, alternative is a micro-mirror array or anyother device capable of spatial modulation to generate a pixel array.Alternatively a variant of an acousto-optical modulator could be used aswell. Different devices will suit different arrangements andapplications.

The dynamic nature of the image generation device is important not onlyfor the changeable nature of the images but the rate at which it can bechanged. In the example, illustrated in FIG. 2 the printed circuit board30 is to be irradiated with laser light of sufficient intensity toinduce a photo chemical reaction in a resist applied to the surface ofthe printed circuit board.

The rate of scrolling must be in concert with the rate at which theimage on the dynamic image generation device is being displayed that isused to modulate the laser output from the CuBr laser.

A control circuit shown in functional block diagram form in FIG. 3achieves in but one embodiment, control of this rate. The details ofthis figure will be described later in the specification.

It can be noted that in prior LDI systems, it is typical for theirradiation of an image to be achieved pixel by pixel or a row of pixelsat a time. With the proposed arrangement, it will be possible todramatically decrease the time it takes to irradiate each PCB thusincreasing the PCB rate of manufacture because it will be possible toproject many thousands of pixels on to the PCB at each laser pulse.

It will also be possible, but it is not illustrated in FIG. 2, toilluminate an area of the PCB. It is envisaged that an array of areasover the PCB surface can be reproduced on the image device so that whenthe PCB board is moved in a suitable X-Y pattern it will ensure that theappropriate areas are illuminated and accurately aligned with adjacentareas.

The embodiment disclosed in FIG. 2 shows that an optical amplifier 18,which is part of the system commonly referred to as a Master OpticalPower Amplifier (MOPA), used to amplify laser radiation. It is onlypreferable to use such a device in the embodiment. The MOPA oscillatorproduces high quality illumination of the modulation device the outputfrom the modulating device (many thousands of pixels) is then amplifiedby the amplifier tube (18) the brightness amplification is typically tenthousand times.

Furthermore in this embodiment, an optical system is shown at thelocation of the output of the MOPA. This arrangement is but one exampleof an arrangement suitable to manipulate the image into a shapepreferable for projection on to a printed circuit board while meetingthe mJ/cm² exposure requirement and wave length of the chosen resist.

It is not inconceivable that either visible photo sensitive or UVsensitive resists could be used, as the requisite power is available foreither to be used.

In the embodiment shown in FIG. 2, the amplified image is pictoriallyrepresented at 19 entering a cylindrical refractive lens 22 which willreduce an area having 2 dimensional information into a line image of onedimension, although it is not anticipated that the second 2 dimensionalinformation will be lost.

In this embodiment a frequency doubler element 24 is used which is alsoknown as a Second Harmonic Generator (SHG). This non-linear device willoutput image information at twice the frequency and provide a modulatedlaser signal at UV frequencies. Using UV frequencies makes it possibleto use conventional and cheaper resists.

However, it should be noted that the SHG has low efficiency, in thevicinity of 20 to 30 percent, so that increased power generated in theoptical amplifier may be required if the power budget of the overalldevice is to balance and eventually provide for an output radiationintensity of the required mJ/cm².

It is however possible to do without an SHG and use just the visiblelaser light output from the optical amplifier. After some suitableoptical manipulation and appropriate choice of resist, the task ofilluminating a PCB to create a track pattern can still be achieved. Forexample, typical resists are monomer and UV would be used to polymerizethe resist to create a polymer. One disadvantage of such resists is thatpolymers are long chain structures and their length can affect thesmallest size of structure created on the work piece. Thus use of UVlaser energy can have limitations if certain resists are used. Whereas,use of visible laser light may provide a reason to develop a resist thathas a photo-initiator suitable for 511 nm and of 578 nm at the increasedpower levels obtainable by using an optical amplifier.

The pictorial representation provided in FIG. 2 is but one simpleexample of a means to disperse the image data over the width of a PCB.In this embodiment the arrangement comprises a planar mirror 26 and aspreader/dispersion optical element 28 to form a line image.

It is also possible to arrange an appropriately curved mirror or mirrorsto change the direction while correcting aberrations and the like beforeit is projected onto the printed circuit board.

Not shown is a further version of suitable optics that can be arrangedto irradiate a rectangular or square area of the PCB. Co-ordination ofmovement of the resist covered PCB with the dynamic image device isagain controlled by a functional circuit not unlike that shown in FIG.3.

It is understood that the line and space specifications will likely bein the low micron range and that the generous power budget of thearrangement will allow the use of a larger range of resists which willimportantly likely be less expensive and more stable overall allowingfor the further reduction of costs of PCB manufacture.

The ability to feed in and project large numbers of pixels (200Mpixelsper second) in the form of a linear array and to move them swathe likeover the area of the PCB results directly from the use of a high qualityCuBr laser source and CuBr amplifier used in conjunction with a dynamicimage generator that overall produces sufficient energy to initiate areaction in the chosen resist.

The relatively high power of the visible or UV radiation output willallow for faster irradiation times providing an improvement overconventional systems in relation to the number of boards that can beirradiated per hour.

FIG. 3 depicts a pictorial representation of a functional block diagramfor the control aspects of an LDI apparatus. The electronics andcomputer elements of the controller are known in the art so detaileddiscussions as to their workings is unnecessary. There are manydifferent ways to implement such a controller and in the anticipatedcase of very high throughput equipment, dedicated integrated circuitsdesigned for extremely high data throughput (e.g. as mentionedpreviously as much if not more than 200 Mpixels per second data rate)will likely need to be developed.

The following description is very generic and used for illustrativepurposes only. The laser oscillator 10, dynamic image device 16, theoptical amplifier 18, and dispersion optics are controlled via aninterface circuit 32. These devices also provide data about theiroperation via the interface circuit to the Central Processing Unit (CPU)34. Interface circuit 32 is itself controlled by data from a PersonalComputer (PC) 34.

User interface devices comprising a screen 42 and a keyboard 44 areinterfaced to the PC via normal means to control the program to be usedhowever the apparatus can operate autonomously.

Control of the PCB movement can at its simplest level be achieved byusing a conventional X-Y motion system approach. An X-Y motion controlinterface 46 is instructed via a bus or communication protocol by theCPU and controls an X-Y motion system to achieve accurate movement ofthe PCB in concert with the image being provided to the image displaydevice 16.

Registration of the image to the board dimensions during the irradiationprocess is critical and different means and methods to address thisissue are many and varied and well known in the art.

It will be appreciated by those skilled in the art that the invention isnot restrictive in its use to the particular application describedneither is the present invention restricted in its preferred embodimentwith regard to the particular elements and/or features described ordepicted herein. It will be appreciated that various modifications canbe made without departing from the principles of the invention.

Therefore, the invention should be understood to include all suchmodifications within its scope.

1. An optical image amplifier consisting of: an optical radiation sourceconsisting of an operating copper bromide laser tube having a laseroutput; an optical amplifier spaced from said optical radiation sourceorientated so as to receive an output from said optical radiationsource; an active optical transmission or reflective image formingdevice located intermediate said optical radiation source and saidoptical amplifier by which an image can be formed to spatially modulatethe laser output; and an optical focusing means located at the output ofsaid optical amplifier for translating said image or a portion thereofto a surface to be illuminated.
 2. An optical image amplifier accordingto claim 1 wherein the object to be illuminated is a printed circuitboard blank covered with a photo resist sensitive to said laser output.3. An optical image amplifier according to claim 1 wherein said opticalfocussing means produces an array of pixels, at least one pixel wide,representing at least a portion of said image.
 4. An optical imageamplifier according to claim 1 wherein said optical focussing meansproduces an area of radiation representative of a portion of said image.5. An optical image amplifier according to claim 1 wherein said activeoptical image forming device is a computer controlled liquid crystalarray.
 6. An optical image amplifier according to claim 1 wherein saidactive optical image forming device is a computer controlledmicro-mirror array.
 7. An optical image amplifier according to claim 1wherein green or yellow radiation output from the optical amplifier isfrequency doubled to produce an image array in the UV spectrum.